An ion trap can be used to perform mass spectrometric chemical analysis, in which gaseous ions are filtered according to their mass-to-charge (m/z) ratio. The ion trap can dynamically trap ions from a measurement sample using dynamic electric fields generated by one or more driving signals. The ions can be selectively ejected according to their m/z ratio by changing the characteristics of the electric field. Relative abundance of different ionic species can be measured by scanning the characteristics of the electric field and detecting the ejected ions.
A typical mass spectrometer comprises an ionization source to generate ions from a measurement sample, an ion trap, which may be configured to receive ions and to separate ions in space and/or time, an ion detector to collect filtered/separated ions and measure their abundance, a vacuum system, and power source. Traditionally, to effect trapping of ions, buffer gas (or referred to as cooling gas or damping gas, usually helium) may be added to slow the ions down so that the ion trap can capture them and keep them in the trap. The buffer gas may also be inherently supplied with the sample, for example ambient air. Without the buffer gas, the ions may not be cooled sufficiently to be trapped by the electric field contained within the trap.
Recently, there has been a growing interest in miniaturized mass spectrometers. Miniature (or even portable) analyzers are especially useful in applications such as the detection of chemical warfare agents in combat, detection of pollutants in the field, detection of explosives at airport security checkpoints, etc. The portability of such miniature analyzers may be limited if the effect of cooling ions using a buffer gas is used to trap ions. For example, if an external gas tank has to be included, the overall system may be too large, heavy, or complex for field use. As well, the use of a buffer gas to cool ions may increase the gas load on the system such that pumping requirements are increased beyond what would be practical for a portable instrument. On the other hand, without sufficient buffer gas pressure, the ion capture efficiency may be too low. However, if the buffer gas pressure increases, resolution may suffer, especially when using buffer gasses of higher molecular weight.
Alternate architectures, such as quadrupole filter and time-of-flight mass spectrometers may exist that are more adapted to external ionization, however, these architectures do not lend themselves to miniaturization as well as ion traps. However, ions traps may not be suited to external ionization techniques because the distance over which ions are required to be cooled and trapped is relatively small compared to these architectures.
In addition, it is generally difficult for existing systems (e.g., cylindrical traps) to capture external ions due to potential energy and non-zero kinetic energy at the point of entry.
Therefore, it is desirable to develop ion trap systems and corresponding analyzing methods for performing mass spectrometric analysis with improved capture efficiency yet using a minimum amount of buffer gas pressure to cool the ions sufficiently.